Phosphine is a reagent used in the manufacture of light emitting devices (LEDs) by metal-organic chemical vapor deposition (MOCVD) techniques. For example, manufacturers use phosphine to produce AlInGaP (aluminum indium gallium phosphide) LEDs. Unfortunately, sulfur in the high parts per billion (ppb) or low parts per million (ppm) ranges adversely impacts LED properties. For example, even high purity phosphine starting materials can contribute to this sulfur contamination. Hydrogen sulfide is the most common source of this contamination. In order to eliminate this source of contamination, gas manufacturers require an analytical method for measuring hydrogen sulfide (H.sub.2 S) in phosphine (PH.sub.3) on the order of 100 parts ppb or lower to improve the product by purification.
Extrapolating the semiconductor sulfur analysis back to the phosphine gas is not acceptable as a quality control or product monitoring for a phosphine plant. This process is expensive and highly resource-intensive. Total sulfur analyzers based on reduction to H.sub.2 S in hydrogen cannot be used because PH.sub.3 would decompose on the catalyst at the reduction temperatures required. Furthermore, analyzing total sulfur by oxidation in oxygen to SO.sub.2 would be hazardous, because of the flammability of PH.sub.3.
Gas chromatography provides a problematic technique for measuring trace H.sub.2 S levels in hydride gases. This technique, while theoretically possible, is impracticable because separating trace H.sub.2 S from streams of approximately 100 percent PH.sub.3 is difficult due to the closeness of the elution times and "stickiness" of the H.sub.2 S. Furthermore, PH.sub.3 also interferes with the signal from trace H.sub.2 S sensors.
Finally, as far as known, lead acetate paper tape analyzers have never been used with PH.sub.3 service or in the presence of any toxic or spontaneously flammable gas. (Phosphine is extremely toxic, with a TLV--threshold limit value--of 300 ppbv, and gives no odor warning at the TLV. It is also spontaneously flammable in air.) Furthermore, commercial lead acetate paper tape analyzers are unsuitable, since they are not designed with leak-tight components. In "Gas Analysis and Testing of Gaseous Materials", American Gas Association, Inc., N.Y., c1945 (pp.148, 277-278), V. J. Altieri describes a qualitative test for hydrogen sulfide in gases using a filter paper strip soaked in 5 percent lead acetate solution. The process visually compares the darkness of the paper to paper not exposed to the gas. D. V. Moses et al. in "H.sub.2 S Recorder", U.S. Pat. No. 2,232,622, describe a lead acetate paper tape analyzer. An illuminating gas passes through the tape that is drawn through a roller at a fixed rate. The darkness of the stain is read optically with reflected light to a photocell and compared with unexposed tape. This is an early example of instrumentation for determining H.sub.2 S with a lead acetate tape method. Similarly, Moses et al. (DuPont), in "Automatic Gas Analyzer" in U.S. Pat. No. 2,551,281, disclose an improvement of the above invention. This analyzer humidifies the gas and controls both tape movement and exposure time.
Offutt et al. (Standard Oil), in "Method and Apparatus for Analyzing a Reactive Gas", U.S. Pat. No. 2,800,397, disclose an instrument that analyzes air that does not pass through the tape. The instrument uses transmitted light, rather than reflected light, to measure the tape darkening or H.sub.2 S. This instrument humidifies an air sample at a constant temperature. Furthermore, in "Multiple Stream Gas Analyzer", U.S. Pat. No. 2,895,807, Sorg et al. teach humidifying a tape at a relative humidity of 30-50 percent, using a saturated sodium chromate solution. In this process, a sample's gas stream humidifies the tape.
American Society for Testing and Materials, "Standard Test Method for Hydrogen Sulfide in the Atmosphere by Rate of Change of Reflectance", ASTM D4323-84, 1984, established the standard method for determining H.sub.2 S in air. This standard uses a Houston-Atlas lead acetate paper tape analyzer to measure H.sub.2 S in the range of 1-3000 ppbv.
Kimbell, in "Gas Detector", U.S. Pat. No. 3,464,799, describes a technique for using an analyzer in explosive environments. Finally, Kimbell, in "Periodic Sampling Concentration Indicator", U.S. Pat. No. 4,127,780, describes the operation of the Houston-Atlas lead acetate tape hydrogen sulfide analyzer. This process relies on an electronic differentiating circuit to measure darkening rate. The process can determine darkening rate in a period that is substantially linear with hydrogen sulfide concentration.
Unfortunately, these lead acetate analyzers are unsuitable for measuring hydrogen sulfide in hydride gases such as, phosphine. These devices, constructed without leak-tight components are unsuitable because of phosphine's toxicity and spontaneous flammability in air. Furthermore, these devices add moisture to the test cell itself. This moisture could combine with phosphine to form difficult to remove aqueous phosphine solutions. These solutions can decompose or outgas toxic phosphine.
It is an object of this invention to measure hydrogen sulfide in parts per billion in hydride gases.
It is a further object of this invention to moisten a metal acetate tape with water in a controlled manner.
It is a further object of this invention to measure hydrogen sulfide in an accurate and reliable manner.